Heretofore, the relatively high back electromotive force (EMF) generated by certain electrical motors at operating speed has dictated a higher than available electrical power-supply voltage. Consider, for example, the electrical motor which is designated STC DC Brushless Spindle Motor DLD-5XE (part number 4110001880) by the Shinano Tokki General Corporation. The motor is of the three-phase four-pole half-wave-drive outer-rotor type, suitable for use in 5.25 inch disk memory systems (drives). Included with the motor are Hall-effect devices (sensors) which generate commutation signals. The three motor windings (coils) are configured in a (star) Y-configuration in which one end of each of the windings is connected to a motor-winding-common connector pin (node); and, the other (free) end of each of the windings is connected to a corresponding winding connector pin.
A circuit diagram of a recommended controller for use with the motor is included in the motor specifications by the manufacture. The recommended controller is of the type which is referred to herein as "unipolar". More specifically, the motor Hall-effect devices are shown connected to drive a decoder, which drives three power drivers, each of which drive a corresponding one of the motor windings. The decoder is shown to include three 74LS09-type two-input AND gates and three 74LS04-type inverters. One of the two inputs of each of the gates is connected to the output of a corresponding one of the Hall-effect devices. The other one of the two inputs of each of the gates is connected to the output of a corresponding one of tha inverters, the input of which is connected to the output of a succeeding one of the Hall-effect devices. The output of each of the gates is connected to the input of the corresponding one of the power drivers. The power drivers are each shown to include a pair of transistors, which are connected in a Darlington configuration. More specifically, a first one of each Darlington pair of transistors is configured with the transistor base connected to the output of the corresponding one of the three gates of the decoder and the transistor emitter connected to the base of tha other (second) one of the Darlington pair of transistors. The second one of the Darlington pair of transistors is configured with the transistor collector connected to the winding connector pin which is connected to the free end of the corresponding one of the three motor windings and the transistor emitter connected to a power-driver-common node which is coupled to circuit ground by a feedback resistor. Finally, the motor-winding-common connector pin is connected to a DC power-supply voltage source which provides power for the motor. The manufacture recommends that the DC voltage source have a voltage which is between 10.8 and 13.2 volts.
In another, "unipolar", controller, the winding connector pin which is connected to the free end of each of the motor windings is additionally connected to the anode of a corresponding one of three catch diodes. The cathode of each of the diodes is connected to another node, which is connected to the cathode of a 24-volt zener diode, the anode of which is connected to circuit ground.
Although effective, motors driven by the above-mentioned "unipolar" controllers lack the torque and spin-up acceleration of motors driven by controllers of the type which are referred to herein as "bipolar". Such a controller is included in the integrated-circuit device of the type which is designated UC3620 by the Unitrode Corporation and which is described in their Application Note U-113 and their Specifications UC1633 and UC3620. In addition to a decoder (and associated gates), the controller device employs three power drivers. Each of the power drivers includes a pair of transistors connected in a totem-pole configuration with a pair of clamping (catch) diodes. More particularly, the upper one of each totem-pole pair of transistors is configured with the transistor collector connected to a device pin (number 3) for connection to a DC power-supply voltage source to provide power for the motor and the transistor emitter connected to a respective one of three device pins (numbered 2, 15, and 14) for connection to the winding connector pin connected to the free end of the corresponding one of the three motor windings. The other (lower) one of the totem-pole pair of transistors is configured with the transistor collector connected to the emitter of tha upper transistor and the lower transistor emitter connected to a device pin (number 1) to be coupled to circuit ground by a current-sensing feedback resistor. One of the diodes, a Schottky diode, is configured with the diode cathode connected to the collector of the upper transistor and the diode anode connected to the emitter of the upper transistor. The other one of the diodes is configured with the diode cathode connected to the collector of the lower transistor and the diode anode connected to circuit ground. It is important to nota that the DC power-supply voltage source is connected to (pin number 3 and, thus,) the collectors of the upper transistors of the totem-pole pair of transistors; and, tha motor winding common connector pin is not connected (left floating). Unfortunately, to power the above mentioned motor, the DC power-supply voltage source (connected to pin number 3) should have a voltage which is between approximately 16 and 26 volts.